The present invention relates generally to a filter, and more particularly to a filter used for an exposure apparatus that utilizes an extreme ultraviolet (“EUV”) light emitted from a pulsed light source.
A conventional reduction projection exposure apparatus uses a projection optical system to project a circuit pattern of a mask (or a reticle) onto a wafer, etc. to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in the photolithography technology.
The minimum critical dimension (“CD”) to be transferred by the reduction projection exposure apparatus or resolution is proportionate to a wavelength of the light used for the exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the finer the resolution becomes. Along with the recent demands for the finer semiconductor devices, use of a shorter wavelength of ultraviolet (“UV”) light has been promoted from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to a KrF excimer laser (with a wavelength of approximately 248 nm) and an ArF excimer laser (with a wavelength of approximately 193 nm).
However, the lithography using the UV light cannot catch up with the rapid advancement of the fine processing of a semiconductor device, and a reduction projection optical system using the EUV light with a wavelength of 10 to 15 nm shorter than that of the UV light (referred to as an “EUV exposure apparatus” hereinafter) has been developed to efficiently transfer a very fine circuit pattern of 0.1 μm or less.
The EUV light source typically uses a laser produced plasma (“LPP”) light source and a discharge produced plasma (“DPP”) light source. The LPP light source irradiates a laser light to the target material, generates plasma and the EUV light. The DPP light source circulates a gas around the electrode, discharges it, and generates plasma and the EUV light. Both light sources supply a gas, such as Argon (Ar) and Helium (He), near a plasma generating point, and prevents the high-energy plasma from damaging the optical element, such as a condenser mirror. The degree of vacuum in a light source chamber that accommodates the plasma is maintained at about 1×10−1 Pa.
On the other hand, an illumination optical system chamber that accommodates multilayer mirrors of an illumination optical system should be maintained at the degree of vacuum of 1×10−5 Pa or smaller. This is because any contaminant adheres in a membranous state to the multilayer mirror in the illumination optical system chamber, onto which the EUV light is irradiated. The contaminant adhered multilayer mirror shifts a phase of the EUV light reflected on the contaminated spot, remarkably deteriorating the imaging performance.
Due to a four-digit pressure difference between the light source chamber and the illumination optical system chamber, they cannot be connected as they are. Accordingly, use of differential pumping and a thin film are proposed to connect the light source chamber to the illumination optical system chamber. See, for example, Japanese Patent Application, Publication No. 2000-89000 and Proc. SPIE Vol. 5751, pp. 78-89 (May 2006). The differential pumping method connects these chambers via a small opening between them, and exhausts both chambers with a cylinder-capacity vacuum pump, thereby creating a differential pressure. A method that uses a thin film vacuum-separates the light source chamber from the illumination optical system chamber, and has been conventionally used in the synchrotron radiation pilot plants.
Nevertheless, these methods that use the conventional differential pumping and thin film are not directly applicable to the EUV exposure apparatus. For example, the differential pumping method cannot make the opening enough small that connects the light source chamber to the illumination optical system chamber because the opening is used to allow the EUV light to pass it, and thus cannot create a large pressure difference.
On the other hand, the EUV light intensity is so high that the thin film is not endurable in the thin film method. For example, the EUV exposure apparatus requires about 120 W for the EUV light intensity for the exposure wavelength (referred to as “in-band”) . It is said that the overall intensity of the wave range that contains wavelengths other than the in-band is five to ten times as high as the in-band intensity. When the light has a diameter of 10 mm at the intermediate condensing point, the light intensity is 800 to 3,000 W/cm2 at the intermediate condensing point.
The EUV light is such a low transmittance to materials that the thin film should be thinner than 1 μm. For example, a 0.2-μm thick thin film made of zirconium (Zr) that has the highest transmittance to the exposure wavelength of 13.5 nm can transmit only 50% of the EUV light. The EUV light having the wavelength of 13.5 nm is seldom reflected on the thin film. Therefore, when the high-intensity EUV light enters the thin film, the thin film absorbs 50% of the incident energy, and the temperature rises. When the energy of 20 W/cm2 is incident upon a Zr thin film having a thickness of 0.2 μm and a diameter of 10 mm, and the film absorbs 50% of the energy, the maximum temperature becomes 1,800 K even when the heat radiation is considered. Although this temperature does exceed the Zr's melting point, the Zr evaporation speed is about 2×10−3 μm/hr, and the 0.2-μm thick Zr film rapidly thins and cannot maintain the vacuum diaphragm function. Thus, when the thin film requires a small incident intensity of the EUV light but this intensity is insufficient to the exposure and the thin film cannot used in reality.
In addition, a self-sustainment such a thin film as 1 μm or thinner is very difficult in a large area. The 0.2-μm thick film can be self-sustained only in a size of about 1 mm or smaller. One known method of supporting the thin film is a method of forming a thin film on a mesh structure. For example, a transmission electron microscope (“TEM”) irradiates an electron beam onto a sample on an organic film that transmits the electron beam. Since the organic film is so thin as 0.01 μm to 0.1 μm, the self-sustainment of the organic film is difficult and the mesh structure is used to support the organic film. The synchrotron radiation experimental device uses a metallic thin film supported by the mesh structure. The irradiation intensity can be considered time-wise uniform in both the TEM and the synchrotron radiation experimental device. The synchrotron radiation is a repetition of pulses. However, a ratio between the pulse's time width and period is not so large and the frequency is so high as 1 GHz to 1 MHz, and thus the synchrotron radiation can be regarded as the continuum. On the other hand, the plasma light source has a lower repetitive frequency, e.g., about 10 kHz, than the synchrotron radiation, and such a large pulsed width as 10 to 50 nsec, and cannot be regarded as a continuum.